In order to shrink the package size of an LCD module, it is often desirable to mount the LCD integrated circuit (IC) driver directly to one of the glass substrates used to fabricate the LCD display. Wire bond or flip chip methods may be used to attach the IC die on to the glass substrate, with the flip chip technique being typically preferred because it requires less space. When wire bonds are used, additional area and volume is needed for both the wire bonds and either the lid enclosure or the polymeric glob top encapsulant material. Flip chip attachment requires that a suitable bond be formed between the integrated circuit (IC) pad metallization and the substrate metallization. For flip chip applications, the IC bond pads are either gold plated or tin-lead bumped using a well known controlled collapse chip connection (C4) process. Examples of direct chip bonding to the glass substrate are demonstrated in U.S. Pat. Nos. 4,643,526 and 4,917,466, incorporated herein by reference.
It would be advantageous if one could bond directly to the transparent indium tin oxide (ITO) used for the display electrodes, however, most present bonding techniques such as thermo-compression do not provide sufficient adhesion for providing reliable joints. It is usually necessary to selectively add subsequent metal layers such as nickel/gold over the ITO through additive vacuum deposition and follow this by plating steps to achieve a bondable surface. Presently, this can only be accomplished through time consuming and repetitive photolithography operations to metallize the desired areas at a significant expense.
Alternatively, significant effort has been spent trying to attach IC's directly to the ITO metallization pattern using conductive epoxies and anisotropic conductive films. While these methods have merit, they do have limitations, especially as package densities increase and conductor lines are routed between bond pads, leaving geometries on the order of 0.0254 millimeter (0.001 inch) or less. A need thus exists in the art for a method for forming metallic pad areas on a substrate during array fabrication which, if desired, may be overplated prior to polarizer attach to provide high resolution bond sites for "chip on glass" applications.